1. Technical Field
The invention relates to composite abrasive products of the type having an active part, consisting of a compact containing grains of ultra-hard product (diamond or cubic boron nitride) representing more than 80% percent per volume of the compact, each grain being bonded directly to its neighbours so that the compact has a polycrystalline structure, and a hard and refractory support integrally bonded with the compact, formed essentially by a refractory carbide such as tungsten carbide.
The term "compact" designates a sintered product formed by grains bonded together by bridges, created by diffusion of material in the plastic state. Such sintering in the plastic phase is obtained at pressures and temperatures of the same order of magnitude as the pressures and temperatures used to synthetize diamond grains, when the compact is diamond based, and those required for transforming hexagonal boron nitride into cubic boron nitride (CBN) if the ultra-hard product is CBN.
Numerous applications require thermostable abrasive products, i.e. products which can be heated to high temperatures during use thereof without rapid destruction. Much work has been carried out for about fifteen years for providing the compacts themselves, then composite abrasive products incorporating them, with such a thermostable character.
2. Prior Art
Composite products of the above-defined type have been known for a very long time. French patent No. 2,089,415 describes a composite product formed of a diamond compact on a tungsten carbide support, the compact and the carbide containing a same additive which acts as a catalyst for converting carbon into diamond and as a sintering binder, which may be nickel or cobalt.
Such a product is not suitable for cutting under conditions which raise it to a high temperature. The cobalt-containing polycristalline diamond compact is not thermostable. The interface between the compact and the support, formed of tungsten carbide, diamond and binder, does not resist under such conditions, even when a metal barrier has been inserted during manufacture between the support and the compact.
In an attempt to make the compact thermostable, it has been proposed to eliminate the catalyst by leaching (U.S. Pat. No. 3,745,623). But the residual compact is porous and has a very large specific area, so that its life duration at high temperature is reduced by oxidation, and its resistance to abrasion, even at ambient temperature, is reduced due to the absence of an intergranular bonding phase.
Attempts have been made to overcome this drawback either by infiltrating a ceramic phase into the pores of a leached compact or by creating a ceramic phase from the metal phase, comprising at least a catalyst, during manufacture. But this improvement, although it makes it possible to give the compact itself a thermostable character, does not solve another problem which, apparently, has not been completely appreciated up to the present invention.
Whether the compact is made of CBN rendered thermostable due to the nature of the binders which it contains, or whether it is made of polycrystalline diamond rendered thermostable by substituting a ceramic phase for a metal catalyst, it has been found that, when the product mounted on a support is subjected, during use, to high temperatures, there is failure of the interface due to the stresses. Now, for numerous applications, for example ground drilling, the abrasive product is subjected to such stresses that it is desirable for it to withstand them at high temperature.